When you connect to components with a balanced line, you are essentially connecting their chassis together.
Now imagine that instead of both pieces of gear being tied to the same ground bus (like we always tell electricians to do, and sometimes they listen), they are connected to two separate ground buses.
Further imagine that one ground bus has a few volts of voltage potential and the other one does not. Plug that XLR cable in and you now have current flowing over that cable between the amp and mixer.
What happens when you send 60-cycle current next to an audio line? You hear a 60-cycle hum. What happens when the shield of your balanced audio cable is carrying a few volts of 60-cycle current? A really well defined 60-cycle hum.
Some think the easiest way to solve this problem is to decouple the two electricity ground buses by using a “cheater” plug on either the amp or the mixer. Cheater plugs disconnect the safety ground connection, and in some instances will solve a ground loop.
However, it’s very dangerous and you should never be do this. Will do so kill someone every time you do it? No. But do you want to be the one who eliminated the safety ground when someone does get shocked or killed? Me neither. Don’t do it.
The right solution is to make sure all your audio equipment is grounded to the same ground bus. Electricians will argue about this sometimes if they don’t understand why, but don’t relent.
As part of a proper system, all of your audio equipment should be powered from the same panel (that is preferably powered by a ground isolated transformer). This is why doing the electrical work in a properly designed install can be a little more expensive than just running lights and outlets; all the conduit runs have to go back to the same spot rather than wherever it’s convenient. But I digress…
Now, if you can’t re-work your electrical grounding system, and you still have a ground loop, there is still a temporary solution. You can lift the ground on the balanced audio line on one end or the other. Most of the time, this will stop the ground. Doing this can theoretically leave you open to RF and EMI noise getting into the line as the shield is not shunted to ground on both ends, but in practice it’s usually a better alternative to the 60 cycle hum.
Sometimes ground loops will show up when connecting things like keyboards to a mixer using a DI (direct box). Better DIs will have a ground lift switch that disconnects pin 1 (shield) inside the DI and will usually break the ground loop. If your DI doesn’t have that feature you should get new DIs. If you can afford that, you can make up a short XLR cable that has pin 1 open at the female end. Make sure you clearly identify this cable as a ground lift.
There’s so much more I could say about ground loops, but this is already getting long. And to all those sticklers for detail, yes I know I glossed over some points and over-simplified things to make the concepts understandable. If the concepts in this post are new to you, I encourage you to spend some time researching them. There is a ton of information available on the web; start Googling ground loops and spend a few hours educating yourself. Your efforts will be rewarded.
Mike Sessler is the Technical Director at Coast Hills Community Church in Aliso Viejo, CA. He has been involved in live production for over 20 years and is the author of the blog, Church Tech Arts . He also hosts a weekly podcast called Church Tech Weekly on the TechArtsNetwork.